Conventional domestic ovens, such as gas ranges and electrically heated ovens, are based on direct cooking. These ovens are bulky and costly for cooking a small amount of food, since they consume a large amount of power, which is a disadvantage. Recently, many combined ovens have been introduced. Among these is that disclosed in U.S. Pat. No. 4,350,874, in which there is described a cooking chamber formed of a case part and base part. The base part has a step-like wall inside which fits the case part; there is a heating chamber containing a heating element; and there is a hot-air fan in the heating chamber in order to transfer heat. There is also a cooling fan placed adjacent to the heating chamber. In this cooking oven, the hot-air fan cyclones the air into the cooking chamber at a high speed, in order to distribute the heat evenly onto the food. Thus, the thermal efficiency is quite low. Also, in order to cyclone the hot air, the hot-air fan must operate at a high speed, and the motor service life is short, which is problematic. Also, the hot-air fan is situated almost in-line with the top of the case part and, therefore, the cycloning hot air is directly in contact with the case part, where the temperature of the cooking chamber is high. Thus, during the retrieval of the cooked food, the hand may be burned.
In U.S. Pat. No. 4,817,509, there is disclosed a circular top, which has in its center an air fan. A heating element is placed over the fan. In this cooking oven, hot air is also cycloned into a cooking chamber by a hot-air fan, in order to distribute the hot air evenly onto the food being cooked. This oven also has a low thermal efficiency, and it requires a cooling means to cool the hot-air fan. Hence, the construction becomes complex, which is a disadvantage. More-over, the top of the upper part of this cooking oven, which is contacted by the hot air from the hot-air fan, is flat, and the lower part of the air-fan is almost situated on the same plane. Hence, the top of the upper part may be melted or the hand may be burned during the retrieval of the cooked food. The fan is located below the heating part, and is exposed to the cooking chamber; thus, the splashing of food onto the fan is inevitable, and, in order to clean it, the unit must be dismantled, which is inconvenient.
In U.S. Pat. No. 5,165,328, there is disclosed a cooking chamber that is formed by an upper sealing cylinder, and extension rim part, and a lower sealing cylinder. The unit also includes a cooking-chamber heating means, and an upper support part, as well as a hinged support assembly consisting of an extension part and a lower part. In this cooking oven, the hot air directly contacts the upper sealing cylinder, and the upper part may melt or the hand may be burned; in order to clean the exterior cooking vessel, the air fan must be detached, which is inconvenient. Also, the expander ring is a unitized object, and it is cumbersome to store and is damaged easily. Alternatively, an expander ring consists of two parts for alleviating such problems. The two parts are assembled, however, by aid of small clips, which can be lost when disassembled, and, in addition, the expander ring is cumbersome to assemble. In this cooking oven, the upper support part is attached onto the housing, so that these parts are inseparable, thus making cleaning difficult.